code-quality batch (T-112)

Seven small consultant findings, one commit:

1. TreeSitterLib stores last dlopen error + path in static fields
   instead of swallowing them. Callers that observe a null instance
   can now read the diagnostic.

2. Drop the Cmsghdr alias in libc.dart — back-compat shim with no
   callers; CLAUDE.md forbids those in a solo repo.

3. Drop EditorController._events field + the unused_field
   suppression. The constructor still subscribes via `events.on<...>`
   for _eventSub; the field itself was speculative retention.

4. Replace inline hex / errno literals in native_pty.dart with
   PosixErrno.{eintr,ebadf,epipe} and new libc.{pollin, pollAnyErr,
   sighup, sigkill, sigwinch}. PosixErrno gains eintr.

5. ExtensionManager records activate/deactivate exceptions in a
   `_failed` map exposed as `failedExtensions` + `didFail(id)`.
   Listeners are notified on entry/exit; cleared on a clean
   activate. UI surfaces the degraded state instead of pretending
   everything is fine.

6. file_tree_view imports FileEntry via the clide.dart barrel
   instead of `package:clide/src/files/listing.dart` directly — the
   leak the consultant flagged (barrel already re-exports it).

7. test_app branch in main.dart wrapped in `if (kDebugMode)` so
   release tree-shaker elides the test harness from shipping
   binaries. Source import stays; tree-shake handles the rest.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/builtin/editor/src/editor_controller.dart

@@ -17,13 +17,11 @@ import 'package:clide/kernel/kernel.dart';
 import 'package:flutter/foundation.dart';
 
 class EditorController extends ChangeNotifier {
-  EditorController({required this.ipc, required DaemonBus events}) : _events = events {
+  EditorController({required this.ipc, required DaemonBus events}) {
     _eventSub = events.on<DaemonEvent>().listen(_onEvent);
   }
 
   final DaemonClient ipc;
-  // ignore: unused_field  — kept for future subscription changes
-  final DaemonBus _events;
 
   StreamSubscription<DaemonEvent>? _eventSub;